The present invention involves an aerosol manufacture facility, operation of that aerosol manufacture facility, and automated control of various operations involving the aerosol manufacture facility for producing a particulate product.
Powdered materials are used in many manufacturing processes. One large use for powders is for thick film deposition to prepare films of a variety of materials. Some thick film applications include, for example, deposition of phosphor materials for flat panel displays, and patterning of electrically conductive features for electronic products.
For thick film applications, and for other applications, there is a trend to use powders of ever smaller particles. Generally desirable features in small particles include a small particle size; a narrow particle size distribution; a dense, spherical particle morphology; and a crystalline grain structure. Existing technologies for preparing powdered products, however, often could be improved with respect to attaining all, or substantially all, of these desired features for particles used in thick film applications.
One method that has been used to make small particles is to precipitate the particles from a liquid medium. Such liquid precipitation techniques are often difficult to control to produce particles with the desired characteristics. Also, particles prepared by liquid precipitation routes often are contaminated with significant quantities of surfactants or other organic materials used during the liquid phase processing.
Aerosol methods have also been used to make a variety of small particles. One aerosol method for making small particles is spray pyrolysis, in which an aerosol spray is generated and then converted in a reactor to the desired particles. Spray pyrolysis systems have, however, been mostly experimental and unsuitable for commercial particle production. Furthermore, control of particle size distribution is a concern with spray pyrolysis. Also, spray pyrolysis systems are often inefficient in the use of carrier gases that suspend and carry liquid droplets of the aerosol. Moreover, spray pyrolysis systems are frequently operated in batch mode, and there is a significant potential for inefficiency during transient periods during the early and late stages of particle production. During these periods, variations in particle properties may degrade the quality of the entire batch.
There is a significant need for improved manufacture techniques for making powders of small particles for use in thick film and other applications.
It is an object of the present invention to provide an aerosol method of manufacture suitable for commercial production of particles. Is also an object to provide an aerosol method to produce a particulate product at a high yield and of a high quality. It is also an object to provide an aerosol method involving significant process control for efficient operation, especially for batch processing. It is another object of the present invention to provide an aerosol method that is at least partially automated, to improve efficiency and yield. It is a further object to provide an aerosol manufacture facility in which the aerosol method may be conducted. These and other objects of the present invention are addressed by the present invention as described herein.
In one aspect, the present invention provides an automated aerosol method for processing batches of precursor liquid to manufacture batches of particles of a selected composition. The method involves automation of at least a portion of the process, with automated features being controlled at the direction of an electronic processor that processes instructions for manufacture of the particles of the selected composition. The method is often operated in batch mode. In that case, the batch processing begins with batch initiation operations, during which aerosol generation is commenced, proceeds through intermediate operations, during which the bulk of particle production occurs, and ends with batch termination operations, during which aerosol generation is terminated. As used herein, batch mode refers to processing of a discrete quantity, or batch, of a precursor liquid prepared in a single preparation. The batch mode processing of the present invention includes processing which might be considered as semi-batch or semi-continuous operation because of the length of the batch run involved and/or the manner of product removal. Batch mode processing includes, during the intermediate operations, generating, in an aerosol generator, an aerosol stream from carrier gas supplied and precursor liquid supplied to the aerosol generator and processing the aerosol stream through an aerosol heater to form particles of the selected composition. In a preferred embodiment, droplets of the aerosol are produced, in the aerosol generator, from a reservoir of circulating precursor liquid that overlies a plurality of ultrasonic transducers, which energize precursor liquid in the reservoir to produce the droplets.
In one embodiment of the automated aerosol method of the present invention, an operator instructs the electronic processor to direct processing of a precursor liquid batch to prepare particles of a selected composition. The electronic processor then processes instructions concerning manufacture of particles of the selected composition and, based on the instructions, the electronic processor directs, during batch initiation operations, automatic control in the aerosol manufacture facility of one or more of commencement of precursor liquid supply to the aerosol generator, commencement of carrier gas supply to the aerosol generator, commencement or increase of heat input into the aerosol heater, and activation of ultrasonic transducers in the aerosol generator. During intermediate operations, the electronic processor directs automatic control of one or more of carrier gas supply to the aerosol generator, precursor liquid supply to the aerosol generator, and heat input into the aerosol heater. During batch termination operations, the electronic processor directs automatic control of one or more of deactivation of the ultrasonic transducers, termination of carrier gas supply to the aerosol generator, termination of precursor liquid supply to the aerosol generator, and reduction or termination of heat input into the aerosol heater. In a preferred embodiment, all of these noted operations are automatically controlled at the direction of the electronic processor.
The method of the present invention includes significant flexibility to accommodate automation in a variety of different processing embodiments. For example, the method may include automated cooling of one or more process stream or piece of equipment during the method. In one embodiment, the aerosol stream, after passing through the aerosol heater, passes to an aerosol cooler where a cooling gas is mixed into the aerosol stream to lower the temperature of the aerosol stream, to permit subsequent collection of the particles, the supply of the cooling gas to the aerosol cooler being automatically controlled at the direction of the electronic processor. In one embodiment, the aerosol generator includes a pathway for circulation of a cooling liquid adjacent to ultrasonic transducers to cool the ultrasonic transducers during operation. The cooling liquid pathway is typically interposed between the reservoir of precursor liquid and the ultrasonic transducers, so that ultrasonic signals energizing the precursor liquid first pass through the cooling liquid. Supply of the cooling liquid is automated at the direction of the electronic processor. In another embodiment, a cooling liquid is supplied to the vicinity of electronic driver circuits driving the ultrasonic transducers to cool the circuits, with the supply of the cooling liquid being automatically controlled at the direction of the electronic processor. In another embodiment, a cooling liquid is supplied to end caps adjacent entrance and exit ends of the aerosol heater, with supply of the cooling liquid being automatically controlled at the direction of the electronic processor.
In one aspect, the present invention addresses a significant problem of precursor liquid tending to become more concentrated over time when aerosol generation is from a recirculating precursor liquid. The precursor liquid includes at least one precursor material dissolved or suspended in a liquid vehicle, typically water. Over time, the precursor liquid tends to become more concentrated in the precursor material. This concentration of the precursor liquid over time can result in an undesirable lack of uniformity in properties of particles that are produced. The present invention addresses this problem through the addition of additional liquid vehicle to the aerosol manufacture facility, during generation of the aerosol stream, in a manner to at least partially counteract the tendency of the precursor liquid to otherwise become more concentrated. The additional liquid vehicle may be added, for example, to the aerosol generator to the liquid supply system and/or to the carrier gas supply system.
In one embodiment, the liquid supply system includes two liquid containment tanks, or vessels, to facilitate control of the precursor liquid concentration in the liquid supply system and regulation of supply of the precursor liquid to the aerosol generator. A first, larger vessel acts as the primary supply vessel for the precursor liquid, and a second, smaller vessel acts as a control vessel. During generation of the aerosol stream, precursor liquid is transferred from the first vessel to the second vessel. Precursor liquid is then supplied to the aerosol generator from the second vessel. Effluent precursor liquid from the aerosol generator is returned to the second vessel for recirculation. Additional liquid vehicle may be added to the second vessel to at least partially offset the tendency of the precursor liquid to become more concentrated in the precursor material over time.
Furthermore, in one embodiment control of the concentration of the precursor material in the precursor liquid is automated. For example, the electronic processor may monitor, at some location in the precursor liquid supply system, a property or properties of precursor liquid indicative of the concentration of the precursor material in the precursor liquid at that location. Based at least in part on the monitored property or properties, the electronic processor then directs automatic addition, as necessary, of additional precursor liquid to the precursor liquid supply system to at least partially offset the tendency of the precursor liquid to become more concentrated over time. A convenient location to monitor the property or properties is in the second vessel or in the precursor liquid stream being supplied from the second vessel to the aerosol generator.
In another aspect, the aerosol manufacture method of the present invention addresses detrimental effects on particle quality of transient conditions that may occur during manufacture, and especially during initial stages of particle production during batch processing. The effects of process transients occurring during the initial stages of particle manufacture are, at least in part, addressed with the present invention by conditioning equipment of the manufacture facility during batch initiation operations, prior to particle manufacture. During the conditioning, the temperature of certain equipment is increased to simulate conditions that will exist later during steady state particle manufacture during intermediate operations. The conditioning involves flowing a carrier gas, prior to particle production, through the aerosol heater at an elevated temperature to simulate temperature and flow conditions that will exist when the aerosol stream is flowing through the aerosol heater during steady state particle manufacture. In a preferred embodiment, the heated carrier gas exiting the aerosol heater then passes through the aerosol cooler, where it is mixed with cooling gas and conditions the aerosol cooler. Following the aerosol cooler, the mixture of cooling gas and carrier gas then flows through the particle collector to condition the particle collector. With respect to the aerosol generator, conditioning may include, in addition to flow of the carrier gas, heating the precursor liquid supplied to the aerosol generator prior to commencement of aerosol generation. The heating of the precursor liquid simulates heating that occurs during aerosol generation due to operation of the ultrasonic transducers.
In one aspect, the present invention provides an automated facility for aerosol manufacture of particles according to the method of the present invention. The facility includes an aerosol generator, capable of producing an aerosol stream from carrier gas and precursor liquid, a carrier gas supply system capable of supplying carrier gas to the aerosol generator, a precursor liquid supply system capable of supplying precursor liquid to the aerosol generator, an aerosol heater capable of heating the aerosol stream to form particles of the desired composition, and an electronic processor capable of processing instructions concerning manufacture of particles of the selected composition and capable of communicating, for the purpose of automated control, with one or more of the aerosol generator, the carrier gas supply system, the precursor liquid supply system and the aerosol heater during manufacture of particles in the facility.